Download 46_Muscle contraction

Muscle Contraction
Muscular system
Skeletal Muscle Fiber Structure
Muscle Fiber Structure
Figure 12-4: T-tubules and the sarcoplasmic reticulum
SKELETAL MUSCLE
• Skeletal muscle is made up of
hundreds of muscle fibers
– Fibers consists of threadlike myofibrils
– Myofibrils composed of smaller
myofilaments
– Striations reflect the overlapping of
muscle filaments
Skeletal Muscles Structure
• Muscle are composed of bundles of muscle fibers,
which in turn are made of bundles of myofibrils.
Muscle fiber:
• Sarcolemma: the plasma membrane with inward
extensions form T tubules.
• Sarcoplasm: refers to the cytoplasm.
• Sarcoplasmic reticulum: the ER in muscle.
• Myofilaments actin and myosin, which are
organized into contractile units called
• Sarcomeres: basic units connected end-to-end by
Z- line to form myofibrils.
The Myofilaments
The thick filaments and the thin filaments.
These two filaments are arranged within the
sarcomere in an overlapping manner.
Thin filaments are composed of the protein actin,
the helical backbone of thin filament.
Each actin protein contains an active site which
interacts with the myosin head.
Two other proteins are present in the thin
filaments, tropomyosin and troponin.
Thick filaments are composed of a myosin. The
head extends out from the filament forming cross
bridges which interact with the thin filaments
Sarcomere
– Contractile unit
– Actin (thin) filaments
– Myosin (thick) filaments
Steps in muscle contraction
– Acetylcholine released by a motor
neuron combines with receptors
on the surface of a muscle fiber
– Calcium ions released from the
sarcoplasmic reticulum
– Calcium ions bind to troponin in
the actin filaments causing the
troponin to change shape
– Troponin pushes tropomyosin
away from the active sites on the
actin filaments
– ATP binds to myosin
– ATP is split, putting the myosin head
in a high-energy state
– Energized myosin heads bind to the
exposed active sites on the actin
filaments
– The actin filament is pulled toward the
center of the sarcomere
– Myosin head binds a new ATP
– Myosin head detaches from the
actin
– Myosin reattaches to new active
sites so that the filaments are
pulled past one another
– Muscle continues to shorten
STIMULATION
• Contraction of skeletal muscle is initiated
when an action potential traveling down a
motor neuron reaches the neuromuscular
junction.
• Motor neuron releases acetylcholine into
synaptic cleft, which binds with receptors on
muscle fiber.
• Depolarizes (change in electric charge) the
sarcolemma of the muscle fiber.
STIMULATION
• Depolarizes (change in electric charge) the
sarcolemma of the muscle fiber.
• This action potential travels down the
inward-projecting T tubules that reach deep
into the muscle fiber.
• Depolarization of T tubules opens calcium
channels in the sarcoplasmic reticulum.
• Causing the to release of stored calcium
ions.
• Ca2+ then diffuse into the myofibrils and
bind to troponin complex, which change
its shape.
• Pushing tropomyosin away from the
active sites on the actin filament.
• Expose myosin-binding sites, which are
capable of interacting with myosin
heads, forming cross bridges after ATP
breakdown to ADP& Pi.
• A new ATP binds to myosin heads,
breaking the cross bridges and myosin
detach from actin.
• Tropomyosin then covers active sites on
the actin molecules and relaxation
occurs.
• After contraction, ACH inactivated, the
Ca2+ moves back into the sarcoplasmic
reticulum.
Muscle
contraction
Myosin head (H) attaches to actin filament (A),
forming a crossbridge.
Providing energy for muscle
contraction
– ATP hydrolysis provides the
energy to “cock” the myosin
– Creatine phosphate is used for
intermediate energy storage
– Glycogen is the fuel stored in
muscle fibers
Antagonistic action of
skeletal muscles
–
–
–
–
Agonist muscle contracts
Antagonist muscle relaxes
Groups of muscles work together
Series of separate stimuli timed close together
produces a smooth, sustained contraction
Muscle action
Tetanus
Excitation-Contraction Coupling
• Provides the physiologic mechanism
whereby an electrical discharge at the
muscle initiates the chemical events that
cause activation.
• When stimulated to contract, Ca++ released
from SR.
• Rapid binding of Ca++ to troponin in actin
filaments releases troponin’s inhibition of
actin-myosin inhibition.
• Actin combines with
myosin-ATP.
• Actin activates ATPase,
which splits ATP.
• Energy release produces
crossbridge movement
(power stroke).
• New ATP attaches to
myosin crossbridge to
dissociate from actin.
Relaxation
• When muscle stimulation
ceases, intracellular Ca++
decreases as Ca++ is
pumped back in SR by
active transport.
• Ca++ removal restores
inhibitory action of
troponin-tropomyosin.